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Goal III: Enable Human Exploration
Send comments to:
Dr. Paul Abell
SBAG Goal III Lead
Paul.A.Abell@nasa.gov
Comments due by:
09/19/2015
Also feel free to contact Dr. Nancy Chabot, SBAG chair:
Nancy.Chabot@jhuapl.edu
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Theme 1: Human Mission Target Identification
Summary
Identification of human mission targets within the small body population involves several stages: (1)
Evaluation of astrodynamical accessibility (required mission change-in-velocity (v), required mission
duration, available launch dates, etc.); (2) Evaluation of relevant physical characteristics (e.g.,
composition, shape, size, rotation rate, presence of secondary or tertiary bodies, etc.); and (3)
Evaluation of relevant human factors (e.g., health and safety in the small body’s environment, effects
of space environment on crew during the mission duration, etc.). Small bodies exhibit a broad range of
physical characteristics, such as rotation rate, orientation of spin axis, and presence of secondary or
tertiary objects. These quantities can offer advantages, challenges, or pose hazards to spacecraft and
crew (Table 1). Therefore, a set of criteria defining what ranges of parameter values are acceptable for
human missions must be established. Criteria on the suitability of a given NEA as a human destination
based on the NEA’s physical properties and human factors will evolve over time as planned crew
infrastructure and space exploration architectures evolve. However, astrodynamical accessibility
criteria can generally be evaluated independently and is taken as the starting point for identifying NEAs
that are candidate targets for human missions. Many of these NEAs are highly accessible and offer
opportunities that have significant advantages over other destinations (Figure 1). Identification of
human mission targets is most effectively achieved using an infrared space survey telescope.
Objective 1: Discover and identify asteroids that are astrodynamically accessible
from Earth
NASA’s Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) is an ongoing
project (Barbee et al., 2013) with the goal of monitoring the growing known NEA population for
mission accessibility. The list of known NHATS-compliant NEAs is maintained at
http://neo.jpl.nasa.gov/nhats/, and is automatically updated daily as new NEAs are discovered and orbit
estimates are updated. However, the NHATS list of potential mission targets should not be interpreted
as a complete list of viable NEAs for an actual human exploration mission. As new observations of
these objects are obtained, the NEA orbits are updated, which can change the viable mission targets and
their mission parameters. Physical characteristics, discussed further below, can also significantly
restrict the total number of suitable targets. Additionally, tighter constraints on other criteria, such as
round-trip mission duration or v can shrink the number of targets considerably (See Tables 1 and 2 in
Barbee et al, 2013). Because of these factors, it is beneficial to continue to discover new asteroids to
increase the pool of potential targets. However, ground-based visible light surveys are biased against
objects with long synodic periods and low albedos. This limits the number of NEAs that may be found
in highly accessible orbits. The most effective method for finding these objects is via a space-based
infrared system. Therefore the most important aspect to this objective is:

Identifying NEAs in Earth-like orbits. What are the number of highly accessible targets with
Earth-like orbits? What is their size frequency distribution? What is the range of albedos of
this population?
Necessary precursor measurements: Deployment of a space-based infrared survey telescope.
Applied exploration science research: Continuation of the NHATs project. Discovery and
characterization of more NHATs-compliant targets.
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Figure 1. A mission to a near-Earth asteroid can require less propulsion and a shorter mission duration
that a human mission to any other celestial target. Less than 1% of the most accessible asteroids are
currently known (yellow circles), but a dedicated survey (filling in the yellow-hatched region) would
reveal abundant asteroid stepping-stone opportunities as a gateway for interplanetary exploration.
Objective 2: Expand the knowledge of asteroid physical characteristics
High accessibility of an NEA alone does not make for an attractive target; it is also important to know
the physical characteristics of an asteroid. For example, some asteroids have rotation periods of less
than a minute (Miles, 2008), which would prove challenging for crewed and robotic missions. There
are many techniques that may be used in concert to investigate an asteroid (Table 1). Radar
observations can dramatically improve the accuracy of our estimates of a NEA’s orbit, and these data
can help constrain the composition of the object (i.e., metal). In addition, NEA diameters and spin rates
can be derived from radar data. High-resolution radar images can be inverted (and, in some cases,
combined with lightcurves) to produce estimates of NEA shapes and spin axes. Such shapes and
rotational information can give insights about surface stability and structure. Radar images can also be
used to identify the presence of asteroid moons (Benner et al., Asteroids IV, 2015). Spectroscopy can
constrain surface composition and infrared measurements can determine asteroid surface reflectivity
(albedo), and size (Mainzer et al, Asteroids IV, 2015). Lightcurve measurements can determine spin
rate. Thermophysical modeling combines many of these datasets (size, shape, spin axis), to produce an
average surface thermal inertia of a body, which helps constrain surface characteristics. However, only
a small fraction of NEAs have been studied with any one of these techniques, and an even smaller
fraction has been studied using multiple techniques. By filling in these knowledge gaps, we would have
a better sense of the type of environment a mission would encounter. This would allow us to better
prepare for such a mission, well before a specific target is chosen. More objects need to be studied
with these techniques in order to:
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Understand the physical characteristics of asteroids. What types of compositions are present
among the NEA population? What is the range of NEA shapes and rotation states? What are
their surfaces like? Do they have companions? What characteristics make a good target?
Identify the best methods to characterize NEAs for human exploration. What techniques,
or combinations of techniques, give the most relevant data needed to inform human missions?
Necessary precursor measurements: Investigation of potential target NEAs via robotic spacecraft.
Applied exploration science research: Continued characterization of asteroids using established groundbased techniques. Modelling of NEAs using ground-based and spacecraft data.
Theme 2: Understand How to Work on or Interact with the Small Body Surface
Summary
Detailed knowledge of the surface properties of small bodies in addition to the physical and mechanical
properties of the near-surface and interior must be obtained prior to conducting human explorations
missions on these objects. Such data are crucial for planning science (optimizing tools and techniques)
and resource utilization activities that will be conducted at small body targets. Use of a robotic
precursor mission is the ideal method to obtain this information. The knowledge needed for small body
interaction depends upon the level and degree in which the interaction takes place. The following
categories describe the different levels of interaction in increasing order of complexity: 1) Approach; 2)
Transient Contact; and 3) Extensive surface interaction with anchoring
Objective 1: Characterize the environment for extended proximity operations
A prerequisite to interacting with the surface of a small body is approaching it safely. The small body's
rotation state must be understood and the rates within limits for human and spacecraft interaction. The
rotation period must be predictable across timescales greater than the duration of the mission so that the
dynamic and lighting environments can be accounted for in operations planning. The orbits and
rotation states of any satellites/particulates must be known, and safe approach and departure corridors
identified. In addition, the crew will no doubt conduct a variety of operations over an extended period
of time, necessitating accurate positional information with respect to the small body’s surface and any
other objects (e.g., co-orbitals) nearby. This would involve detailed knowledge of the gravitational
field of the object. Many small bodies are not spherical objects and often have irregular shapes and
mass concentrations. The gravitational field albeit weak, will not be uniform.
Therefore the following are important to this particular objective:

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Understanding the rotation state of the object. How fast is the object spinning? Is this a nonprincipal axis rotator? How does the axis of rotation and the spin rate affect the operations that
can be conducted by the crew?
Identification of co-orbitals or particulates in proximity to the object. Does the object have
a companion? Are there particulates in close proximity to the object? If so, where are they with
respect to the object?
Mapping the gravitational field of the object. If the gravity field uniform? Are there
variations with rotation? Do stable orbits exist and where are they located?
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Necessary precursor measurements: In situ high resolution imagery of the specific target in question to
determine rotation state and presence of co-orbitals. Detailed radio science mapping of the target’s mass
distribution and gravity field.
Applied exploration science research: Ground-based optical and radar observations of select targets.
Modeling of lightcurves. Models of co-orbital generation and dynamical evolution. Modelling small
body orbital dynamics.
Objective 2: Characterize the small body’s surface physical characteristics
The first approach to the surface of a small body for a human mission might be conducted
cautiously via telepresence of a small robotic vehicle, or with a piloted vehicle utilizing test-firings of
braking and attitude-control thrusters to confirm findings from the previously deployed precursor
spacecraft. Simple transient contact is the safest and easiest interaction, a touch-and-go that requires
only forces directed away from the surface. A push on the surface itself can provide that force.
Spacecraft, robotic vehicles, sample collectors, or spacewalkers can do this. During the brief contact
with the surface they may be able to collect a variety samples or deploy equipment. Designing and
deploying these assets will depend on advance knowledge of the target’s physical characteristics.

Understanding the surface response to spacecraft thrusters or mechanical interaction.
What is the surface like in terms of regolith? Is there a significant amount of particulate
material? What is the cohesion of the particles? How easily are they liberated from the surface?
 Understanding the local gravity environment. Are there any areas on the body that have near
zero or negative local gravity (including rotational effects)? Will this help or hinder touch and
go operations?
 Determination of composition. What is the composition of the object? Does the object have
more than one type of composition? Is the composition detrimental, benign, or beneficial for
human interaction?
Necessary precursor measurements: In situ high resolution imagery of the specific target to determine
surface morphology, composition, and particle size distribution. Detailed radio science mapping of the
target’s gravity field locally with respect to rotation. Investigation of the surface via small payloads.
Applied exploration science research: Modelling small body surface compositions and regolith dynamics.
Experiments conducted in regolith simulants under micro-gravity conditions, ISS experiments of
meteoritic materials.
Objective 3: Characterize the small body’s near-surface geotechnical and
mechanical properties
Touchdown of a spacecraft to the surface of a small body can be challenging, but it has been repeatedly
demonstrated (e.g., NEAR-Shoemaker, Hayabusa, Philae lander). However, attaching a spacecraft or
instrument to the surface for extended operations and interactions requires knowledge of the mechanical
properties of the near-surface. This is required to plan for a spacewalking astronaut as well, whether
moving on pre-deployed lines or nets, articulated booms or arms attached to spacecraft, or on small
maneuverable spacecraft. Designing the systems required for extended periods of operation at the surface
and possibly while anchored to the subsurface will depend on detailed knowledge of the target’s
geotechnical, mechanical, and internal properties.
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Understanding how to anchor spacecraft, astronauts, and instruments to the small body
surface. What are the forces required to anchor? Are there particular techniques that are
beneficial for human exploration? Is there a need to anchor in all instances? Can anchors be
deployed in regolith or at boulders?
 Understanding how to translate across the small body surface. What are the best ways to
translate for an astronaut on EVA vs. a spacecraft? Will regolith help or hinder this activity?
Are there preferred locations/conditions for translation?
 Understanding how to collect samples from the small body. What types of samples can be
collected? How difficult is it to collect sub-surface samples or samples from a boulder?
 Understanding how to minimize contamination of work sites, equipment, and habitat.
What protocols need to be implemented? How to clean suits and equipment?
Necessary precursor measurements: Remote sensing and in situ investigations of the surface via a variety
of payloads. Payloads that measure surface and subsurface properties such as particle size distribution,
internal structure, cohesion, compaction, shear, porosity, etc. would be required.
Applied exploration science research: Modelling small body surface and sub-surface properties, regolith
depth and evolution. Experiments conducted in regolith simulants under micro-gravity conditions, ISS
experiments of meteoritic materials.
Theme 3: Understand the Small Body Environment and its Potential Risk/Benefit to
Crew, Systems, and Operational Assets
Summary
Understanding the nature of the small body environment and the associated risks and potential benefits
to human explorers is important to facilitate future exploration and proximity operations at/near small
bodies. In general, unknowns relating to human operations risk factors for the small body environment
can be most effectively addressed through one or several robotic precursor missions. These “known
unknowns” can be placed into three categories: 1) Understand the small body particulate environment;
2) Understand the ionizing radiation environment at small body surfaces, and; 3) Understand the
internal structure and tectonic stability of small bodies.
Objective 1: Characterize the small body particulate environment
Dust in small body environment may act as both a hazard and a nuisance, especially given the known
physical, cohesive, and electrostatic properties of dust in a microgravity environment. There are also
potential health concerns relating to dust particle morphology (e.g., jagged shapes). Dust, if defined by
electrostatically-dominated particles, can actually be much larger than equivalent terrestrial or lunar dust.
Characterizing the nature, sources, and behavior of dust in the small body environment is therefore a key
mid-term (within the next 5-10 years) objective that will feed into future hardware trades. Of particular
importance are:

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Understanding the expected particulate environment due surface disturbance due to
micrometeoroid impacts and human operations. How much material is ejected into space,
and how does it behave following ejection, and does the potential for adhesion to spacecraft
and/or astronauts pose a substantial risk?
Understand charged particle levitation following surface disturbances.
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Understand possible dust and gas emission via sublimation from volatile-rich objects.
Phobos/Deimos: Additionally, understand the population of the particulate torus associated with Phobos
and Deimos potentially produced by micrometeoroid impacts and material ejected from Mars.
Necessary precursor measurements: High-phase angle, long-duration imaging *in-situ* (including
during and following impact-induced surface disturbance) of small bodies. Dust environment detector
similar to those carried by legacy ALSEP experiment packages and (more recently) the LADEE
spacecraft.
Applied exploration science research: Modeling and impact laboratory experiments; ISS experiments,
mitigation experiments and strategy development.
Objective 2: Characterize the small body radiation environment
This includes both secondary charged particles and neutrons produced in the regolith. Ameliorating
radiation effects through hardware and mission design choices lessens the need to use pharmacological
remediation strategies for human exploration. In addition, small body surfaces may afford a measure of
radiation shielding that could provide benefit during long duration exploration missions. The CRaTER
instrument on Lunar Reconnaissance Orbiter (LRO) continues to provide new information about the
radiation environment in cislunar space that can potentially be used to inform the unique aspects of the
small body environment that require further measurements, which include:
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Understand local effects on the plasma and electrostatic environment from solar flare
activity. The concern here is that solar flares may lead to enhanced dust levitation or other
hazards/nuisances.
Understand small body surfaces as a source of secondary radiation. Small body surfaces
may have enhanced radiation return during solar flares, and laboratory measurements have not
shown to be a fully adequate measurement.
Necessary Precursor Measurements: Instrument with analogous capabilities to LRO CRaTER should be
flown to a target object to characterize the small body radiation environment. Of particular importance
is measuring the degree of shielding provided by a small body during a solar flare and from galactic
cosmic rays, though guaranteeing such a measurement may require a long-duration mission.
Applied Exploration Science Research: Laboratory modeling; data mining of XGRS/GRS instruments
from Dawn, Hayabusa, and NEAR; extrapolation from LRO-CRaTER dataset, modeling secondary
radiation using lunar examples. Existing radiation models need to be upgraded to fully accommodate
planetary regoliths (including small bodies) as a source of secondary radiation, as well as potential
interactions between the small body and the spacecraft.
Objective 3: Local and Global Internal Stability of Small Bodies
Considering the diversity of small bodies, a “one-size-fits-all” model for asteroid interiors is largely
infeasible: every small body is different. However, we can at least come up with broad categories of
internal structure given enough information. This raises the importance of adequate precursor mission
characterization to understand the internal structure and stability of small body surfaces. Of particular
concern is the potential effects of human operations that interact with the surface, which could cause
mass wasting; understanding the stability of small bodies is thus also important to enable small body insitu resource utilization. Given the evidence that many smaller asteroids are rotating at or near breakup
speeds, it is certainly possible that relatively small surface disturbances could lead to major
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reorganization of material, including shedding.
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Understand the local structural stability of small bodies. Limited astronaut interactions, such
as geologic sample collection or the emplacement of subsurface seismic instrumentation, could
potentially activate mass movement of material.
Understand the global structural stability of small bodies. Larger-scale ISRU efforts that
extensively interact with (for example) small body regolith could have unpredictable
consequences, and these must be more fully understood.
Necessary Precursor Measurements: Measure rotation rate of target asteroid to better than 1% precision;
measurement of mass using radio science; measure the volume and shape model using LIDAR; in-situ
measurement of cohesion, shear strength suing imaging and geotechnical experiments, in-situ
seismometer or seismometer network. Emerging radar and muon tomography measurements could offer
a potential pathway to map asteroid internal structure prior to surface interactions and their use should
be more fully explored. OSIRIS-REx and Hayabusa2 will also provide important observations pertaining
to the surface cohesion of representative small bodies, particularly the effects of surface disturbances.
Applied Exploration Science Research: Conduct long-term light curve and radar observations.
Conduct analog research on the ISS to determine cohesion and shear strength in an appropriately weak
gravity field, particularly for cohesion and shear strength testing. Forthcoming LEO cubesat experiments
(AOSAT I) as well as microgravity flights and suborbital experiments might also provide useful
information.
Theme 4: Small Body Resource Utilization
Summary
Human activity in the solar system is necessarily limited because of the historical requirement that all
fuel, shielding, equipment, life-support, supplies, and vehicles for any given activity be transported
from the surface of the Earth at great expense. To expand human activity beyond cis-lunar space, the
cost must be dramatically reduced. The identification, recovery, and utilization of resources from nearEarth objects (NEOs) represent the best opportunity to achieve this goal and should be a central
objective for future space exploration agencies and entities. The promise of NEO in situ resource
utilitization (ISRU) has been discussed and written about for decades. Achieving the ISRU objectives
will provide the information required to make an informed technical assessment of the costeffectiveness and practicality of NEO ISRU for the support of human spaceflight. Carbonaceous
chondrites contain ~1-20% water by mass [ref], in some cases up to 40% recoverable HCNO volatiles
(4, ck ref on HCNO). Water can be broken down and used as propellant directly [ref] or in a thermal
propulsion system as reaction mass (5). Water from NEOs has also long been contemplated for lifesupport and radiation shielding (Lewis ref in 80s). Phobos and Deimos, which may be captured
asteroids, have also long been considered as sources of propellant in Mars orbit. Since Phobos and
Deimos appear to be dry, at least at the surface, an examination of their subsurface compositions will
be necessary to determine their ISRU potential. In addition, NEOs are potential sources for valuable
platinum-group metals, as well as other metals that could be useful for construction in space [ref].
Objective 1: Identify at least 100 asteroids brighter than H = 25 (D ~ 50m, Pv ~
0.06) with round trip delta V < 6 km/s from Low Earth Orbit
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Continuing astronomical surveys are vital in order to identify many potential asteroid targets, because
some will be unsuitable (e.g. due to rotation rates) and the long synodic period means that only a small
fraction will be accessible in any given year. It is insufficient to merely discover asteroids; they must be
characterized to determine whether they are carbonaceous. This can be via albedo measurement and
spectral analysis. The most effective way to conduct this survey would be to deploy a dedicated spacebased NEO survey telescope away from Earth’s vicinity.
Objective 2: Test hardware to excavate and mechanically process asteroid material
(or suitable simulant) and convert it into propellant in a microgravity, vacuum
environment
There are many unknowns about the bulk mechanical properties of asteroid material and what would be
required to excavate and process it into material suitable for further processing (which might be
thermal and/or chemical) into useful resource materials. This is further complicated by unknowns
associated with the execution of such mechanical and chemical processes in microgravity and vacuum.
The best platform for developing and testing these processes in an asteroid-like environment and
making them robust may be the International Space Station.
Objective 3: Characterize the surface and near-surface composition and
geotechnical properties of a NEO resource target
While we have hand samples from NEOs in our collections of meteorites, the bulk properties of NEOs
are relatively unconstrained. The spectral properties of the asteroid (2008 TC3) did not predict the
geochemical diversity within and across the Almahata Sitta meteorites collected. This raises questions
about the extent to which a meteorite sample may be representative of the bulk properties of its parent
NEO. This need to be resolved via in situ surface and subsurface studies of a target NEO in addition to
characterization of its geotechnical properties.
Objective 4: Characterize the surface and near-surface composition and
geotechnical properties of Phobos and Deimos
Phobos and Deimos have long been suggested as sources of in-space propellant for missions to land
astronauts on Mars and provide fuel for return to cis-lunar space. However, there is no evidence from
surface spectroscopic observations of the presence of water resources. This may not reflect their
subsurface compositions. The potential value of such resources motivates a spacecraft mission focused
on the Martian moons to determine their subsurface compositions at depth and geotechnical properties.
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Table 1: Important physical characteristics relevant to human exploration of asteroids.
Rotation rate
Asteroids can have rotation periods ranging from
Measurement techniques
hundreds of hours (Pravec and Harris, 2000) to less than
 Lightcurves
a minute (Miles, 2008). Fast rotators present several
 Radar observations
challenges:
 Humans experience physiological difficulties
when in a fast-rotating frame
 A quickly spinning asteroid my be at its
cohesional strength limit, any disruption may
dislodge debris
 Synching spacecraft with a fast-rotating object
could be operationally expensive
Asteroids with rotation periods greater than two hours
are preferable.
Rotation axis
Although most asteroids have a stable rotation axis,
Measurement techniques
some undergo a “tumbling” motion in which the rotation
 Radar observations
axis changes over time (e.g., Takahashi et al., 2013).
 Lightcurves
 Chaotically tumbling asteroids may present
operational challenges
Stable rotation axis, or predictable rotation axis
alignment, is preferable.
Presence of satellites
Roughly 16% of near-Earth asteroids have a satellite
Measurement techniques
(Margot et al. 2002). Triple systems (asteroids with two
 Radar observations
satellites) have also been observed.
 High-resolution imaging
 The presence of a moon allows for the
determination of asteroid density, and therefore
mass. Prior knowledge of these properties could
greatly simplify mission planning and reduce
mission risk.
 Asteroid moons are often tidally locked, and
therefore often spin at the same rate as the parent
body. They could be attractive mission targets in
their own right.
 A moon could also act as a dangerous projectile
and will need to be avoided.
The area around a target asteroid should be searched
for satellites.
Cohesion and stability
Some asteroids are monoliths, solid pieces of rock or
Measurement techniques
metal. Others are loosely bound aggregations of dust and
 Radar shape modeling
rock, and are called “rubble-piles” (e.g. Fujiwara et al.,
 Lightcurve modeling
2006).
 Shape and rotation rate.
Advance knowledge of the type of gravitational and
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physical environment would assist mission planning.
Mass
Asteroid mass is a highly valuable quantity for mission
planning. It is also difficult to measure.
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
Thermophysical modeling
can provide some
constraints on if the
surface is coasted in
regolith or dust.
Measurement techniques
 Can be derived from
natural satellite orbit
 Flyby/rendezvous mission
 Some constraints from
combining Yarkovsky
measurements and
thermophysical modeling
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